This invention relates to air heating devices, and in particular to an electric air heater having a fixed volume airflow with damper control to determine the heat output.
Conventional, electric air heaters share many of the same concepts, such as:
1. Almost all heaters employ Nichrome type heating elements in one form or another, which are heated to extreme temperatures. Some such heaters used forced air and others use natural draft or convection for heat transfer.
2. Almost all conventional heaters utilize electrical or electro-mechanical switching to deenergize the heating elements for control of heat discharged. Such controls normally take the form of thermostats or similar elements.
3. Whenever an abnormal condition arises, such as when a motor fails to deliver air or air discharge is physically blocked, almost all heating devices depend on some kind of thermo-limiting device to deactivate the heating elements to prevent a fire or electrical breakdown. This can be accomplished through the use of limit controls or some other form of thermally active limiting device. Often such controls or devices become very complex, even to the extent of employing secondary backup controls in the event that the primary control also fails to function.
4. Most conventional air heaters that are permanently installed are comprised of assemblies which must be fitted together to form the final device, from simple enclosures with cover plates, to segmented devices which are joined on site during installation. As a result, some devices are simple to install while others are complex, requiring skilled, knowledgeable installers for proper installation.
Conventional electrical air heating appliances use heat sources which can often reach temperatures as high as 1600.degree. F., well above the combustible limit of most nonmetallic materials. The safe operation of such heaters is dependent on the operation of limit devices which turn off the heating elements during an abnormal situation. In contrast, in conventional PTC (positive temperature coefficient) heaters, the possibility of overheating and fire is not present. PTC elements are self-limiting as far as temperature of the elements. This self-limiting phenomena is the function of the physical chemistry of the doped ceramic material of which the PTC element is made.
The typical PTC element has an internal fixed value of resistance based on a given temperature. As the temperature rises or falls, so does the resistance of the PTC element. Typically, at around 340.degree. F., the resistance of an element rises to a point to completely eliminate the flow of current through the element, thus stopping any further temperature rise of the element. This self-limiting feature will avoid an overheating condition (and consequential possible fire), and is the primary difference between a conventional heating source that employs wire heating elements and one which employs PTC elements.